Biomaterials are a class of functional materials designed to interact with and become incorporated into the human body for uses such as prostheses. Unlike products obtained through bioengineering, the manufacture of biomaterials rarely requires cellular processing or a biological intermediary.
There is a need for biomimetic structures friendly to body chemistry and physiology. Goals for these biomaterials are that they possess mechanical stability for hardness, compressive strength, flexural strength, and wear resistance, controlled microstructure to develop functional gradients, controlled interfacial properties to maintain structural integrity in physiological conditions, and well-understood surface chemistry tailored to provide appropriate adhesion properties, chemical resistance, long implant life, and patient comfort.
A wide variety of biomaterials exist such as biocompatible polymers and bioceramics. Biocompatible polymers include biodegradable polymers for use in providing structural support to organs and other body parts, drug delivery, and the like, and non-biodegradable polymers such as polymer prosthesis. For example, hip joint replacements typically make use of non-biodegradable polymers. The technique typically requires a traumatic in vivo polymerization reaction within the cup of a hip joint, and the use of a metal ball joint within the cup which can result in stress shielding (described below), causing bone dissolution. Uneven wear rates between the metal ball joint and the polymer sockets can cause the polymer to disintegrate within the body causing even more rapid dissolution. As a result, the interface between the metal ball joint and bone often loosens over time causing the patient great discomfort. The result is that hip joint replacement using current state-of-the-art technology may have to be performed more than once in a patient.
Bioceramics have found widespread use in periodontic and orthopedic applications as well as oral, plastic, and ear, nose, and throat surgery. Common materials for bioceramics are alumina, zirconia, calcium phosphate based ceramics, and glass-ceramics. Bioceramics can be categorized according to their in vivo interaction, typically as bioinert, bioactive, and resorbable bioceramics. Various types of bioceramics undergo fixation within the body according to different processes. Some processes are generally more favorable than others, but in many cases a bioceramic material that undergoes fixation within the body via one advantageous interaction may be associated with other disadvantages.
Bioinert bioceramics include single crystal and polycrystalline alumina and zirconia, and are characterized as such because the body encapsulates the ceramics with fibrous tissue as a natural mechanism in recognition of the inert ceramic as a foreign object, and tissue growth associated with this reaction is used to mechanically fix the ceramic article in the body. In dense alumina and zirconia, the tissue grows into surface irregularities. In porous polycrystalline alumina, zirconia, etc., tissue grows into the pores.
Resorbable bioceramics include tricalcium phosphate, calcium sulfate, and calcium phosphate salt based bioceramics. They are used to replace damaged tissue and to eventually be resorbed such that host tissue surrounding an implant made of the resorbable ceramic eventually replaces the implant.
Bioactive bioceramics include hydroxyapatite bioceramics, glass, and glass-ceramics. A "bioactive" material is one that elicits a specific biological response at its surface which results in the formation of a bond with tissue. Thus, bioactive materials undergo chemical reactions in the body, but only at their surfaces. These chemical reactions lead to chemical and biological bonding to tissue at the interface between tissue and a bioactive implant, rather than mere ingrowth of tissue into pores of the implant which provide mechanical fixation. A characteristic of bioactive ceramic articles is the formation of a hydroxycarbonate apatite (HCA) layer on the surface of the article. The degree of bioactivity is measured in terms of the rate of formation of HCA, bonding, strength, and thickness of the bonding layer as well as cellular activity.
Although many ceramic compositions have been tested as implants to repair various parts of the body, few have achieved human clinical application. Problems associated with ceramic implants typically involve the lack of a stable interface with connective tissue, or a lack of matching of the mechanical behavior of the implant with the tissue to be replaced, or both (L. L. Hench, "Bioceramics: from Concept to Clinic", J. Am. Ceram. Soc., 74, 1487-1510 (1991)). In the case of bioinert bioceramic materials, only a mechanical interlock is obtained, and if the mechanical fixation between the surrounding tissue and implant is not strong enough, then loosening of the bioceramic can occur causing necrosis of the surrounding tissue along with total implant failure. For example, when alumina or zirconia implants are implanted with a tight mechanical fit within the body and movement does not occur at the interface with tissue, they are clinically successful. However, if movement occurs, the fibrous capsule surrounding the implant can grow to become several hundred microns thick and the implant can loosen, leading to clinical failure.
Problems long associated with resorbable bioceramics are the maintenance of strength, stability of the interface, and matching of the resorption rate to the regeneration rate of the host tissue. Furthermore, the constituents of resorbable biomaterials must be metabolically acceptable since large quantities of material must be digested by cells. This imposes a severe limitation on these compositions.
The success of bioceramic implants depends upon properties of strength, fatigue resistance, fracture toughness, and the like. These properties are reported to be a function of grain size and purity, but strength typically decreases as grain size increases. High temperature sintering of .beta.-tricalcium phosphate results typically in micron scale grains (Akao, et al., "Dense Polycrystalline .beta.-tricalcium Phosphate for Prosthetic Applications", J. Mat. Sci., 17, 343-346 (1982)). It has been reported that an increase in the average grain size of polycrystalline .alpha.-Al.sub.2 O.sub.3 to greater than 7 microns can decrease mechanical properties by about 20% (Hench J. Am. Ceram. Soc., referenced above). Additionally, as strength is increased, porosity typically decreases according to prior art liquid phase and solid state sintering techniques (Hench, et al., Ed., Introduction to Bioceramics, Chapter 1, pages 17-20 (1993)).
One problem associated with hard tissue prosthesis, for example, artificial bones or bone portions, is "stress shielding". This phenomenon results when a prosthesis of relatively high Young's modulus, such as alumina, is used as an implant against bone. The higher modulus of elasticity of the implant results in its carrying nearly all the load. This prevents the bone from being loaded, a requirement for bone to remain healthy and strong. That is, stress shielding weakens bone in the region where a load applied to the bone is lowest or in compression. Bone that is unloaded or loaded in compression undergoes a biological change that leads to bone resorption. The elastic modulus of cortical bone ranges between 7 and 25 GPa, which is 10 to 50 times lower than that of alumina. The modulus of cancellous bone is significantly lower than that of cortical bone. The modulus of elasticity of a variety of materials used for load bearing implants is compared with the modulus values of cortical bone and cancerous bone in Hench, et al., Ed. Introduction to Bioceramics, referenced above.
Hydroxyapatite, Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2, is an attractive and widely utilized bioceramic material for orthopedic and dental implants because it closely resembles native tooth and bone crystal structure. Though hydroxyapatite is the most common bioceramic, applications for its use have been limited by its processability and architectural design conceptualization. Conventional processing lacks compositional purity and homogeneity. Because hydroxyapatite is difficult to sinter, dense hydroxyapatite structures for dental implants and low wear orthopedic applications typically have been obtained by high-temperature and/or high-pressure sintering with glassy sintering aids which frequently induce decomposition to undesirable phases with poor mechanical stability and poor chemical resistance to physiological conditions. Thus, conventionally-formed hydroxyapatite necessitates expensive processing and compromises structural integrity due to the presence of secondary phases. Existing methods require high forming and machining costs to obtain products with complex shapes. Furthermore, typical conventional hydroxyapatite decomposes above 1250.degree. C. This results in a material with poor mechanical stability and poor chemical resistance.
Jarcho, et al., in "Hydroxyapatite Synthesis and Characterization in Dense Polycrystalline Form", J. Mater. Sci., 11, 2027-2035 (1976)), describe a process for forming dense polycrystalline hydroxyapatite that is "substantially stronger than other hydroxyapatite materials", and that elicits "an excellent biological response when implanted in bone" (p. 2027). A precipitation method was used and material of average grain size of from about 150-700 nm recovered. However, Jarcho, et al. report low volume fraction of pores, and report considerable grain growth during sintering even at firing temperatures of 1000.degree. C. Jarcho, et al. achieved 99% density in some cases, but using a technique that can be impractical for forming desired shapes. M. Akao, et al., in "Mechanical Properties of Sintered Hydroxyapatite for Prosthetic Applications", J. Mater. Sci., 16, 809-812 (1981), report the compressive flexural torsional and dynamic torsional strengths of polycrystalline hydroxyapatite sintered at 1300.degree. C. for three hours and, compare the mechanical properties of the product with those of cortical bone, dentine, and enamel. The compressive strength of the sintered hydroxy apatite was approximately 3-6 times as strong as that of cortical bone.
There is much room for improvement in the use of hydroxyapatite as implants. As reported by Hench et al., "Bioceramics: from concept to clinic", American Ceramic Society Bulletin 72, 4, 93-98 (1993), "Because (hydroxyapatite) implants have low reliability under tensile load, such calcium phosphate bioceramics can only be used as powders, or as small, unloaded implants such as in the middle ear, dental implants with reinforcing metal posts, coatings on metal implants, low-loaded porous implants where bone growth acts as a reinforcing phase, and as the bioactive phase in a composite." (p. 97). Hench, J. Am. Ceram Soc. (1991; referenced above) reports that hydroxyapatite has been used as a coating on porous metal surfaces for fixation of orthopedic prostheses, in particular, that hydroxyapatite powder in the pores of porous, coated-metal implants would significantly affect the rate and vitality of bone ingrowth into the pores. It is reported that many investigators have explored this technique, with plasma spray coating of implants generally being preferred. Hench reports, however, that long term animal studies and clinical trials of load-bearing dental and orthopedic prostheses suggest that the hydroxyapatite coatings may degrade or come off (p. 1504). Thus, the creation of new forms of hydroxyapatite having improved mechanical properties would have significant use, but the results of prior art attempts have been disappointing.
Recently, attention has been focused on nanocrystalline or nanocomposite materials for mechanical, optical and catalytic applications. By designing materials from the cluster level, crystallite building blocks of less than 10 nm are possible, through which unique size-dependent properties such as quantum confinement effect and superparamagnetism can be obtained. Various nanocrystalline ceramics for structural applications have been especially rigorously investigated in the 1990's. R. Siegel discusses nanophase metals and ceramics in "Recent Progress in Nanophase Materials", in Processing and Properties of Nanocrystalline Materials, C. Suryanarayana, et al., Ed., The Minerals, Metals & Materials Society (1996), noting that while many methods exist for the synthesis of nanostructured materials, including chemical or physical vapor deposition, gas condensation, chemical precipitation, aerosol reactions, and biological templating, synthesis and processing methods for creating tailored nanostructures are sorely needed, especially techniques that allow careful control of surface and interface chemistry and that can lead to adherent surface coatings or well-consolidated bulk materials. It is noted that in the case of normally soft metals, decreasing grain sizes of the metal below a critical length scale (less than about 50 nm) for the sources of dislocations in the metal increases the metal's strength. It is noted that clusters of metals, intermetallic compounds, and ceramics have been consolidated to form ultrafine-grained polycrystals that have mechanical properties remarkably different and improved relative to their conventional coarse-grained counterpart. Nanophase copper and palladium, assembled from clusters with diameters in the range of 5-7 nm, are noted for having hardness and yield strength values up to 500% greater than in conventionally-produced metal. It is also noted that ceramics and conventionally brittle intermetallics can be rendered ductile by being synthesized from clusters with sizes below about 15 nm, the ductility resulting from the increased ease with which the ultrafine grains can slide by one another in "grain-boundary sliding." However, synthesis of nanocrystalline or nanocomposite materials is difficult. Significant effort has been put into such synthesis and it is likely that in many or most attempts particle sizes on the nanometer scale are not recovered due to agglomeration. A delicate balance of synthetic parameters typically must be elucidated in connection with a particular set of materials.
In an article entitled, "New Nanocomposite Structural Ceramics", by Niihara, et al., the synthesis and characterization of micro- and nanocomposite structural ceramics is reported. A variety of ceramics including Al.sub.2 O.sub.3 /SiC, Al.sub.2 O.sub.3 /Si.sub.3 N.sub.4, and the like were investigated. Nanocomposites including intra- and intergranular nanocomposites and nano/nanocomposites demonstrated improvement of mechanical properties and/or machinability and superplasticity.
While hydroxyapatite is used widely, and a hydroxyapatite formulation having mechanical and morphological properties advantageous for prostheses would be very useful, attempts to date have failed to produce reliable structural hydroxyapatite implants. Accordingly, it is an object of the invention to provide relatively simple techniques for synthesizing nanocrystalline apatite materials having structural and morphological properties useful for structural implants. In particular, it is an object to provide synthesis techniques that produce densified, nanocrystalline material under mild conditions including relatively low sintering temperature, reducing or eliminating decomposition and minimizing cost. It is another object to obtain apatite materials having enhanced mechanical and chemical resistance by maintaining an ultrafine microstructure in sintering through suppression of grain growth.